The invention relates to a leaf spring arrangement for a chassis of a motor vehicle.
The suspension and damping of a motor vehicle particularly affects the vertical vehicle vibrations. The design of the spring is decisive for the driving comfort, i.e. for the vibration stress on the occupants as well as the cargo. The suspension further plays a decisive role in driving safety. The known suspension elements include leaf springs which are used in automobiles as well as in the commercial vehicle sector. Leaf springs are constructed single or multilayered and in certain applications can also take over the function of wheel control. They enable a favorable force transmission into the vehicle body. The weight of leaf springs made of steel is not insignificant. Especially in the commercial vehicle sector however, weight savings are sought to be realized, in order to carry along as much cargo as possible while not exceeding the permissible total weight. Of course, lighter leaf springs also have a positive effect with regard to fuel consumption. Therefore, the state of the art includes the manufacturing of leaf springs not only from steel but also from glass fiber reinforced plastics (GFK), to save weight.
In rigid axles with longitudinal leaf springs made of glass fiber reinforced plastics the stiffness in Y-direction, i.e. transverse to the driving direction becomes smaller compared to leaf springs made of steel having the same spring width, even though the leaf springs made of GFK have the same stiffness as leaf springs made of steel in Z-direction, i.e. vertical relative to the vehicle.
The greater flexibility in Y-direction however, can have negative effects on the transverse guiding behavior of the vehicle, because a greater transverse offset of the axle results from transverse forces. The greater transverse offset can be counteracted in that leaf springs in which at least one layer is formed by a profiled metal section are used. Such a leaf spring is described in DE 198 147 92 A1. However, profiled metal sections have a higher weight than glass fiber reinforced plastics, so that the desired saving potential cannot be achieved by metal inserts. Further, the manufacturing process of such springs is complex.
JP 540 25 986 A2 proposes to cover an inner layer of a fiber reinforced plastic made of carbon fibers (CFK) on both sides with GFK-fibers, so that this three-layered arrangement has an overall higher strength. The stiffer CFK fibers have a lower weight and due to the higher stiffness or respectively, lower elasticity should be arranged in the region of the neutral fiber of the leaf spring, i.e. primarily in the central region. A significant disadvantage is however, that the CFK materials are significantly more expensive than glass fiber materials. Currently, the factor is about 1:16 for 1 kg of fibers.
DE 101 39 780 A1 discloses a composite leaf spring with improved lateral stiffness. The increase of the lateral stiffness is caused by introducing two carbon fiber inserts into the longitudinal side regions of a glass fiber composite spring. The volume amount of the carbon fiber inserts is preferably between 10 and 20 volume percent of the total volume of the spring and is a function of the load characteristics required in the suspension system. Glass fiber springs constructed as composite leaf springs are lighter compared to conventional steel leaf springs.
GB 2 041 846 A also discloses a leaf spring arrangement, wherein the central arrangement of the leaf spring has a support surface, on which the leaf spring is supported. The support surface is tilted at an angle relative to the vertical axis of the motor vehicle. Due to the horizontally oriented ends and the tilted center connection, the construction of the leaf spring is relatively elaborate because the direction of the force applied in the region of the center connection leads to a force component in Y-direction during deflection in Z-direction.
The invention is based on the object to provide a leaf spring arrangement for a chassis of a motor vehicle which is characterized by a small weight, cost-effective manufacturing and at the same time high stiffness against transverse forces.
This object is attained by a leaf spring arrangement in which a leaf spring is coupled with end sides thereof to a vehicle body by bodywork connections and in the middle region to a vehicle axle of the motor vehicle, by a center connection, wherein the leaf spring includes multiple layers of a glass fiber composite plastic, wherein a reinforcement made of a carbon fiber composite plastic is arranged between an upper layer made of glass fiber composite plastic and a lower layer made of glass fiber composite plastic, wherein at least one reinforcement is arranged along a longitudinal side of a core located between the upper and the lower layer which core is made of a different material than the reinforcements, wherein the center connection of the leaf spring has a support surface on which the leaf spring is supported, wherein the support surface is tilted relative to the vertical axis of the motor vehicle by an angle, and wherein center connections of a vehicle axle are tilted in the opposite direction, wherein the bodywork connection has a bearing axle for the ends of the leaf spring, wherein a plane which is perpendicular to the bearing axis is tilted relative to the vertical axis of the motor vehicle by an angle, and wherein the bearing axles of two leaf springs of a vehicle axle are tilted in the opposite direction.
Advantageous refinements of the invention are the subject matter of the sub claims.
The leaf spring arrangement for a chassis of a motor vehicle according to the invention includes a leaf spring, which is coupled at end sides thereof to a vehicle body by bodywork connections and in its center region to a vehicle axle, in particular a rigid axle of the vehicle by a center connection. The leaf spring is made in a known manner from several layers of a glass fiber composite plastic, wherein a reinforcement made of carbon fiber composite plastic (CFK) is arranged between an upper layer and a lower layer made of glass fiber composite plastic.
At least one reinforcement is arranged along a longitudinal side of a core which is located between the upper and the lower layer, wherein the core is made of glass fiber composite plastic. A reinforcement along a longitudinal side is used which is less expensive than a complete intermediate layer made of CFK. Here, the CFK reinforcement is only present along a longitudinal side, i.e. at a distance to the center longitudinal plane of the leaf spring. The reason is that with regard to laterally acting forces a core made of CFK contributes to a lesser degree to the bending stiffness in transverse direction because the greatest bending tensions occur at a distance to the neutral fiber and not in the neutral fiber. A further increase in lateral stiffness could be realized by a CFK core, wherein however, this measure would be incommensurate with the costs associated therewith when using the material CFK. Therefore, the material CFK is only used in regions of the leaf spring in which the highest bending tensions occur in transverse direction, so that high section moduli can be achieved.
In addition to the adjustment with regard to the material of the center layer of the leaf spring, the leaf spring is oriented overall so that a better transverse guiding is achieved. The center connection of the leaf spring has a support surface for supporting the leaf spring, wherein the support surface is inclined at an angle relative to the vertical axis of the motor vehicle.
The end-side bearing axles in the vehicle body connections are tilted by the angle Gamma. No change of the spring geometry is required. The same leaf springs can be used that can also be used in a horizontal bearing axis or respectively, in horizontally extending support surfaces.
Because the stiffness of the CFK increases the modulus of elasticity compared to leaf springs made of GFK in extension as well as in compression direction, the reinforcements are arranged on both longitudinal sides of the core. The reinforcements can have the same thickness as the core, wherein the thickness of the core or respectively, the reinforcements can vary over the longitudinal extension of the leaf spring. Typically, the thickness of the core as well as the leaf spring increases toward the center connection and decreases toward the vehicle body connections. It is conceivable, that only the thickness of the core or respectively, the thickness of the reinforcements is variable, while the upper and lower layer which is made of GFK has a constant thickness. Usually, the thickness of the GFK spring increases again towards the ends, because at this location a connection to the vehicle body has to be enabled. For this, corresponding bearings can be provided in the ends of the leaf spring.
The reinforcements themselves are each individually smaller than the core and extend preferably over a region from 20% to 50% of the width of the leaf spring.
Theoretically it is possible for the reinforcement to be broader than the bordering layers made of GFK. Because the reinforcement however, is also supported on the GFK layers which border on the upper and the bottom sides and therefore torsion of the reinforcements made of CFK is prevented, it is expedient when the reinforcements do not protrude longitudinally over the upper and lower layer made of GFK. In other words, it is regarded expedient when all layers have the same width.
The angle Gamma in all variants is preferably in a region from 1° to 20°, in particular in a region of from 1° to 5°.